Nothing was said about logging in on a different user ID (I wasn't aware that I was logging in on an ID).

When you start calibre, a configuration folder is created where your user ID preferences are stored. You can find this folder by going to preferences -> advanced -> Open configuration directory (or something like that).

This folder is in theory independent of the library you are using, but in fact the configuration and library are a pair. There are several things in the configuration folder that are library specific. Some that come to mind are saved searched (can depend on custom columns), column layout, and saved sorts.

For various reasons related to how I do development, I want to be able to choose between 4 different configurations/libraries. Because I marry a configuration folder to a library, what I can do is choose between 4 different configuration folders and let the information in the configuration choose the library. I have four command file (shell on linux, bat on windows) that set the CALIBRE_CONFIG_DIRECTORY variable then starts calibre: 'calibre_small.bat' (a tiny library), 'calibre_test.bat' (a larger test library), 'calibre_temp.bat' (a library used for radical experimentation), and 'production.bat' (my real library).

My recommendation is to not use the wizard to switch between libraries, because it isn't designed for that. In particular, it doesn't understand what settings need to change. Instead, set up different command files and switch that way. Starting with itimpi's scripts will help with this process.